1. Field of the Invention
The present invention is directed to both a method and an apparatus for coupling an ATM communication layer to a plurality of mutually independent time-division multiplex communication terminals such that, in particular, all loses are avoided and the variation of the cell delay is minimized.
2. Description of the Prior Art
In the asynchronous transfer mode (ATM), data is transmitted in the ATM layer in cells of 53 bytes (48 bytes payload data and 5 bytes control data) independently of the information they represent (voice communication, data communication, multimedia). The cells are thereby not transmitted continuously but asynchronously, or burst-by-burst dependent, on the momentary demand for transmission bandwidth.
In order to couple such a high-performance ATM layer to terminal points or local networks, it is necessary to divide the cell stream of the ATM layer onto a plurality of time-division multiplex communication terminals (ports) that are independent of one another in terms of time. The problem of decoupling the time-uncorrelated behavior of the ATM communication layer with [sic!] the time-uncorrelated cell transmission demands of the plurality of time-division multiplex communication terminals arises. In order to thereby meet the real-time demands made, for example, of the voice communication, an optimally favorable cell delay variation (CDV) behavior must be assured. That is, the time delay of individual cells should not fluctuate more than a defined value in order to avoid a falsification of the time sequence of cells in the transmission of successive cells via different communication paths.
For a plurality N of time-division multiplex communication terminals, it is known to generate a waiting list having a cell rate corresponding to N-times the cell rate of the individual ports (assuming all ports have the same, typical transmission bandwidth). The cells waiting lists [sic] are then distributed onto the N terminals according to what is referred to as the round-robin method. Those of the N terminals that request a cell at the moment are thereby successively serviced cell-by-cell by the waiting list in a fixed sequence. Since the N communication terminals are independent of one another in terms of time, it can occur that up to N cells are simultaneously requested from the waiting list at a specific point in time. The coupling of the ATM communication layer must thus be able to “buffer” both the ATM bursts as well as the port-side fluctuations in the demand for cells. On the other hand, excessively long waiting lists lead to a deterioration of the cell delay variation behavior of the coupling.
The format and the specifications of the ATM layer are described, for example, in Rathgeb, Wallmeier, “ATM-Infrastruktur für die Hochleistungskommunikation”, pp. 78–90, and the coupling to a plurality of time-division multiplex terminals is described in ATM-Forum, “Baseline Text for Inverse Multiplexing for ATM AF-PHY-0086.000.”
The present invention is, therefore, directed to a method and an apparatus for coupling an ATM communication layer to a plurality of time-division multiplexer communication terminals, whereby cell loses are avoided and the variation of the cell delay is minimized.